The present invention relates to an improved process for the purification of crude terephthalic acid. More particularly, the invention relates to a novel process for recovering terephthalic acid of improved purity from a suspension containing crude terephthalic acid. The crude terephthalic acid is obtained by the liquid phase oxidation of parasubstituted aromatic compounds in the presence of oxidizing catalyst.
In typical known processes for producing terephthalic acid, a para-substituted aromatic compound is oxidized to the desired acid. Such compounds include p-toluic acid, p-tolualdehyde, para-dialkylbenzenes, etc. A preferred dialkylbenzene is para-xylene. The following discussion is directed to the oxidation of para-xylene as an example of such prior art oxidation processes. In the typical oxidation of para-xylene, the oxidation is conducted in acetic acid solvent with molecular oxygen in the presence of a catalyst. The catalyst generally used is a cobalt compound and a manganese compound. An oxidation promoter such as a bromine compound, methyl ethyl ketone, paraldehyde, or acetaldehyde is occasionally used. When para-xylene is oxidized in the liquid phase in acetic acid and in the presence of a catalyst, the product terephthalic acid, being very difficultly soluble in acetic acid, crystallizes out of the solvent acetic acid, forming a suspension. The suspending medium, that is, the acetic acid, contains a small amount of terephthalic acid dissolved therein, catalyst, unreacted para-xylene, intermediate oxidation products such as para-tolualdehyde, para-toluic acid, 4-carboxybenzaldehyde, and other organic impurities which may cause discoloration.
In order to reduce the concentration of such impurities in the solid product, the crude terephthalic acid is generally subjected to a further purification treatment. One such treatment which has been known in the art involves heating the crude terephthalic acid with acetic acid, propionic acid, or butyric acid until it is dissolved, and then cooling the solution to crystallize the terephthalic acid.
Another purification process is disclosed in U.S. Pat. No. 3,431,296. This process involves contacting a suspension composed of 6 to 100 parts by weight of crude terephthalic acid in 100 parts by weight of an aliphatic monocarboxylic acid of 2 to 4 carbon atoms (or a defined aqueous solution thereof) with molecular oxygen-containing gas at 180.degree. to 230.degree. C. in the presence of a cobalt compound. This patent does not contemplate the use of any additional catalyst components.
Other disclosures, e.g., U.S. Pat. No. 4,314,073, U.S. Pat. No. 4,201,871, and Japanese Kokai No. 135,939/74 teach purification processes involving secondary or continuing oxidations (such as that discussed immediately above) wherein the mother liquor from the primary oxidation stage is displaced or diluted by fresh acetic acid. Such secondary and/or continuing oxidations typically utilize catalyst systems comprising cobalt, manganese, and/or bromine components.
U.S. Pat. No. 4,447,646 discloses a purification of crude terephthalic acid which is characterized by the use of a secondary oxidation stage which utilizes a catalyst system comprising cobalt, a bromine compound, and samarium.
It has now been found that the purification of crude terephthalic acid which is produced by the oxidation of para-substituted aromatic compounds can conveniently be accomplished by employing a secondary oxidation stage which utilizes a catalyst system comprising cobalt, a bromine source, and pyridine.